It is a given in business today that it is cheaper to have the most capable entity build world-class goods or provide world-class services instead of having one company try to do all business functions of its business. Payroll, accounting, legal services and even manufacturing have become outsourced to various suppliers whose own business focus is on providing that same service to multiple client businesses. Thus, much electronic manufacturing today is provided by outsourcing most or all of the manufacturing process to various suppliers, manufacturers and assemblers.
Testability features of complex electronic systems are a necessary feature to assure reasonable production results, and therefore manufacturers and designers build and design including capacity for testability into nearly every stage of the production process. With millions or billions of circuits all expected to function together perfectly, designing for testability at nearly every phase of the production cycle has become a way of life. Wafers are tested, chips are tested, and then circuit boards are tested before and after being populated with components. If testing comes late in the manufacturing process, at a populated circuit board level, for example, the board may have already been populated with expensive components before a low level trace is discovered to be an open circuit, potentially causing loss the of the entire board developed up to that point. Thus testing at the lowest level possible is desired. Further, shipping a board that has been tested to another facility where it may be assembled into a larger machine may be an independent cause of the failure of the machine to have satisfactory test results, so testing at the location where the machine or subassembly is assembled may reduce the back and forth discussions over where a failure occurred when shipping is a part of the assembly process. In general, the most recent manufacturing step needs to be verified as having been done correctly before moving on to any subsequent manufacturing steps. This fact affects many related aspects of the downstream manufacturing steps, including diagnostic ability, for example, cost of repair and so forth, are much more complicated.
It should also be recognized that enormous amounts of information transfer between the initiating company and the manufacturing/assembling/testing company typically have to occur in order for a complex electronic product to be assembled and tested satisfactorily. This is a significant cost burden to the process. Further, trained individuals in third party manufacturers show high levels of turnover, getting different jobs in some other company after being trained, thus increasing the loss of sunk costs for this information transfer process. Therefore we have to count on people whose technical abilities are low, and who may be transitory, shift, and production assembly line workers often without specific technical educations.
Accordingly, in the manufacture of very high technology machines such as large scale computer systems, it becomes a significant impediment to successful outsourcing of manufacturing to push significant testing functions into the third party manufacturer/assembler/tester.
This is complicated further because although the design may be a highly prized and confidential element of the finished product, it is important to be able to allow a manufacturer/assembler that is operating as a subcontractor to the owner of the design to provide significant testing functions, or else the boards have to go back and forth before a machine of multiple circuit boards can be assembled with any level of comfort that the finished product will function as intended.
Also, a great many complex circuit boards may go into a machine and they may be of different kinds. Product cycles turn over within every few months. Therefore, with new test fixtures and other hardware to hold them and new programs for each board being required, it becomes very expensive to assemble and maintain test equipment and software for each of the boards. The cost of scraping boards resulting from an inability to repair immediately becomes very high without complete diagnostics. This is at least partly because if one can't sell a new product within months of production it will probably be obsolete and unsaleable.
It should be noted that, at the present time, functional testing is available through subcontracting manufacturers using the targeted environment (i.e., what they refer to as “gold” machines). They also use what is referred to as “rack and stack” equipment (that is, machines of cobbled together boards in a test frame) that mimics the targeted environment on a custom basis to connect signal generators as if it were in the target environment. However, the ability to test all the circuits on the VLSI chips and ASIC chips on circuit boards after they are populated onto the boards is limited to testing at special equipment, not usually available to the subcontracting manufacturers for many or all of the reasons stated above. This slows down and complicates production of large-scale electronic systems. In the subcontracting manufacture's factory, if a unit under test fails in a functional test, there is no real detail presently available at that factory, related to why it failed, or what to fix.
When a board or assembly is assembled, it must be tested to verify correct assembly. Such testing can take any of several forms. One form is Edge Test, in which the tester connects to the board/assembly through the signals that comprise the system-level inputs and outputs of the board/assembly. Often, these signals are at a connector at the edge of a circuit board—that's how the name “Edge Test” comes about. To be practical, edge testing requires the board/assembly to have characteristics that enable an Edge Tester to be general (so that the tester can be reused for multiple types of boards/assemblies) and to be relatively simple (so as to reduce the time and cost for test development and for test application). The desired characteristics are not likely to be present unless certain consistencies are designed into the board/assembly. When designs are consistent in certain fundamental ways, the processes for generating tests and applying them can be automated. In the current era of complicated boards/assemblies, automation is absolutely essential for making testing affordable.
Historically, board test methods have evolved. About three decades ago, when boards were smaller and simpler, testing was commonly done by applying functional test vectors through the I/O pins of the board. Test vectors might have had to be generated manually by someone with knowledge of the functional design.
As boards became more complicated, an In-Circuit Test method evolved in which a fixture had probes that could contact each net on a board. Through the probes, the tester could test individual components, one at a time—a much simpler task than testing multiple components as a combined logical entity.
In recent years, boards have become so dense and complicated physically that more nets are inaccessible to fixture probing. Access is returning to the board edge but new Edge Test methods require the board to be designed to support standard methods for automatically generating test vectors.
Accordingly then, there are several coincident needs which must be attended to in order to take advantage of the economic imperative to outsource manufacturing, while at the same time providing some level of assurance to the owner of the designs that the manufacturing subcontractor will not be able to turn over or otherwise use the benefits of the owner's designs to the owner's competitors (because of the use of proprietary information. Also the training costs to teach them are high for something only for your benefit so they may not want to learn it. Also personnel turnover is high). At the same time, it is of benefit to the contracting manufacturer to be able to cheaply test complex circuit boards of rapidly multiplying variety to ensure continued growth of its outsourcing income.
If there were testers and methods such as are needed, that is ones which limit the amount of redesign and development of software and hardware testing tools another positive aspect is that such an invention will greatly reduces cycle time to ensure that more product gets tested sooner.